In the premature infant the precise interrelationships of essential, exogenously derived hematologic nutrients, i.e., tocopherol(E), selenium(Se), iron(Fe), folate and vitamin B12(B12) are ill-defined. Although the interrelationship between iron and tocopherol is well established, the interaction of Se, the inducer of glutathione peroxidase (GSH-Px), as the additional or supporting cell-sol red cell antioxidant in this system is unclear. Equally unclear are the functional requirements for folate and B12 in maintenance of optimal hematopoietic development because all prior data were obtained independent of the other 3 variables. Feeding practices for premature newborns are, of necessity, exceedingly variable. Thus, this investigation is intended to attack both interaction and requirements in accordance with whatever feeding practies are deemed appropriate for optimal development at a given time. The following regimens will be evaluated: Supplemental folate, supplemental B12, combination of both, and no supplementation. In all infants E and Fe sufficiency will be maintained by early parenteral administration in accordance with accepted practices. The only uncontrolled variable will be Se. A single formula will be used for all infants, many of whom (usually those with birth weights less than 1700 gm) will have received early blood product and/or hyperalimentation support. For all infants attention will be directed to the levels of folate, B12, Se (and E and Fe) in all supporting regimens including formula. The levels of transport proteins will be studied as will maternal nutritional, serological and hematopoietic data. Complete blood counts will be obtained weekly, and plasma levels of E, Se, Fe (bound and total), folate (bound and total), B12 (bound and total), and red cell Se and GSH-Px will be obtained at 2, 4, 6, 8 and 12 weeks and 6 and 12 months by micromethods which utilize no more than 20 ml of blood per patient per study interval. The intensive care nursery and research laboratories are ideally equipped to perform the entire investigation, intended to establish quidelines for optimal hematopoietic development for the premature infant in accordance with all ongoing nutritional efforts.